Ryan Malloy
6881489bf6
What the Raspberry Pi time-server guides get wrong on a Pi 4, with the measurements. The headline artifact is a four-line pps-gpio patch: PREEMPT_RT force-threads IRQ handlers, and pps-gpio takes its timestamp inside its handler, so the realtime kernel puts a scheduler between the electrical edge and the clock. IRQF_NO_THREAD takes RMS offset from 2468 ns to 199 ns. - kernel/ the patch - dashboard/ live status page (position hidden by default) - docs-site/ the write-up (Astro/Starlight, brass, no tutorial section)
19 lines
658 B
Diff
19 lines
658 B
Diff
diff --git a/drivers/pps/clients/pps-gpio.c b/drivers/pps/clients/pps-gpio.c
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index 65d17781d..5ba2aaebe 100644
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--- a/drivers/pps/clients/pps-gpio.c
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+++ b/drivers/pps/clients/pps-gpio.c
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@@ -156,6 +156,13 @@ get_irqf_trigger_flags(const struct pps_gpio_device_data *data)
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IRQF_TRIGGER_FALLING : IRQF_TRIGGER_RISING);
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}
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+ /* The handler timestamps the pulse, so it has to run in hard-irq
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+ * context. Under PREEMPT_RT it would otherwise be force-threaded and
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+ * the timestamp taken after thread wakeup latency, adding microseconds
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+ * of jitter to an edge that should be good to nanoseconds.
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+ */
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+ flags |= IRQF_NO_THREAD;
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+
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return flags;
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}
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